1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to a technique which is effectively applicable to a large-screen or high-definition liquid crystal display device.
2. Description of Related Arts
Conventionally, a liquid crystal display device has been used as various display devices including a display of a personal computer or a television receiver set.
The liquid crystal display device is a device which displays an image (a video image) by applying an electric field to a liquid crystal material which is filled between two sheets of glass substrates and controlling the orientation of liquid crystal molecules in the liquid crystal material so as to control the transmission or interruption of light.
Further, although the liquid crystal display device is classified into various kinds depending on the difference in the orientation of the liquid crystal molecules or the difference in method for applying an electric field, recently, a TFT liquid crystal display device which exhibits a high expression ability of natural colors and can easily achieve the acceleration of a response speed has been popularly used.
The TFT liquid crystal display device is a liquid crystal display device which fills a liquid crystal material between a TFT substrate which arranges TFT elements on a glass substrate in an array and a counter substrate which arranges color filters or the like on a surface which faces a surface of the TFT substrate on which the TFT elements are formed.
Here, in the TFT substrate, for example, gate electrode lines, data electrode lines, drain electrodes, gate insulating films, amorphous silicon (a-Si) films and display electrodes are formed on a glass substrate, and TFT (Thin-Film Transistor) elements each of which is constituted of the gate electrode line, a drain electrode which is branched from the data electrode line, a source electrode, the gate insulating film and the amorphous silicon film are arranged in an array.
Further, in manufacturing the TFT substrate, for example, first of all, a conductive film for forming the gate electrode lines is formed on the glass substrate. An etching resist for forming a pattern is formed on a conductive film and, thereafter, unnecessary portions of the conductive film are removed thus forming the gate electrode lines. Thereafter, in the same manner as steps for forming the gate electrode lines, data electrode lines and the like are formed by repeating a series of treatments such as the formation of a film, the formation of a resist and etching.
Further, in the manufacture of the TFT substrate, in a step for forming the etching resist, conventionally, a resist material is applied to the conductive film, and a resist pattern is formed by the exposure treatment using a mask.
However, the method which performs the exposure using the mask uses a preliminarily designed mask size and hence, for example, when the conductive film for forming the gate electrode lines is formed, even when irregularities exist in film thickness, the gate electrode lines are always formed with a fixed width. Recently, a screen of the liquid crystal display device becomes large-sized and hence, at the time of forming the conductive film, the irregularities in film thickness are liable to be easily generated. Accordingly, when the gate electrode lines are formed with the preliminarily designed width as in the case of the prior art, at the time of forming the conductive film, the difference between a cross-sectional area of the gate electrode line in a region where the film thickness is decreased and a cross-sectional area of the gate electrode line in a region where the film thickness is increased becomes large. As a result, the irregularities of the line resistance of the respective gate electrode lines are increased and hence, there has been a drawback that an image quality of an image (a video image) which is displayed by the liquid crystal display device becomes non-uniform in the inside of a screen, that is, regions having a favorable image quality and regions having a poor image quality are generated in the inside of the screen.
To avoid the occurrence of these drawbacks, conventionally, for example, by taking the irregularities of a film thickness and the width of the conductive film for the gate electrode lines into consideration, to set the line resistance of the gate electrode lines to a fixed value or less in the whole region above the TFT substrate, the film thickness of the conductive film is increased or the width of the gate electrode line is increased. However, when the width of the gate electrode line is increased, a numerical aperture, that is, an area which allows the transmission of light from a backlight and the display of colors is decreased corresponding to the increase of the width of the gate electrode line thus giving rise to a drawback that the luminance is lowered and the performance of the liquid crystal display device is lowered. Further, when the film thickness of the conductive film is increased, a film forming time is increased corresponding to the increased thickness of the conductive film and, at the same time, a quantity of conductor material to be used is increased. Accordingly, there also arises a drawback that a manufacturing cost of the TFT substrate (the liquid crystal display device) is pushed up.